This project proposes to define the bacterial flora of developing gingivitis in humans and the flora of severe periodontitis in young adults. Bacterial samples are obtained at three time points in a 26-day progression from health to gingivitis, and from supra- and subgingival sites affected by severe periodontitis. Randomly selected isolates to represent the entire flora are characterized to the species level. Nearly 4,000 isolates have been characterized to date, which include more than 150 species, nearly 100 of which are previously undescribed. Those encountered to date statistically account for approximately 95% of the variety of species present. Resampling of sites is performed to assess variability, and supragingival samples are used to help assess degree of contamination of subgingival samples by the sampling method. This grant is part of a larger overall effort to definitively characterize the flora of defined periodontal conditions.